There are many instances when it is necessary to separate the components of a gas stream or to provide a stream that is a given purity with respect to a particular gas. For example, many hydrocarbon gases such as natural gas, thermally or catalytically cracked gas, or refinery off gas are contaminated with one or more inert gases which lower their heat content or otherwise impair their marketability. Such inert gases include nitrogen, helium, and argon. Contamination of natural gas with nitrogen is particularly common and it is often desirable to separate the nitrogen from the natural gas.
Likewise, oil may be contaminated with nitrogen that is a natural component or that is derived by nitrogen injections for reviving oil wells, as is common in the central and north Texas areas of the United States. When oil is contamination with nitrogen, the oil producer may be forced to curtail oil production because government regulations prevent him from burning the nitrogen-rich associated gas and both environmental laws and a desire to preserve valuable resources prohibit him from venting the associated gas into the atmosphere. The oil producer is thus limited by the choices of technology available to him for processing the associated gases from an oil well. The prior art technology, which involves cryogenic techniques, cannot economically process the natural gas streams which contain more than 3 mol % nitrogen, even after subsidization with the revenue from oil production.
Olefins such as ethylene and propylene are commonly present in thermally or catalytically cracked gas streams or in refinery off gases. These gases generally comprise methane, carbon monoxide, carbon dioxide, acetylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butenes, butanes, C5""s, C6-C8, non-aromatics, benzene, toluene, xylenes, ethyl benzene, styrene, C9 xe2x88x92400xc2x0 F. gasoline, 400+xc2x0 F. fuel oil, and water. These olefin-containing streams are commonly associated with large quantities of hydrogen.
Numerous processes are known for isolating and recovering olefins from cracked, refinery, and synthetic gases. Some processes utilize specific paraffinic compounds as an absorption oil while others utilize an aromatic absorption oil as a solvent within an absorber column or an absorber-stripper column having a reboiler. Some of these processes additionally isolate a methane-rich stream and/or a hydrogen-rich stream.
It is often desirable to provide a gas stream that is a given purity with regard to a specific gas. A wide variety of gaseous streams are found in petroleum refineries. Some streams are integral parts of a specific process, such as those that are recycled from a fractionating column to a reactor. Such a recycle stream may be an impure hydrogen stream which must be purified before returning to the reactor and/or combining with a make-up hydrogen stream. Other such gaseous streams may be a byproduct of a major refinery process and may be sent to one or more other processes which are nearby and which require a hydrogen feed stream of a specific purity.
As crudes having higher sulfur content and higher carbon-to-hydrogen ratio continue to be processed and as stricter environmental regulations requiring lower sulfur content arise, hydrogen demand is expected to grow. Even though a substantial portion of this increased demand will be met by steam reforming of light hydrocarbons and partial oxidation of heavy hydrocarbons, upgrading of existing off-gas streams is a viable alternative for supplying the needed hydrogen.
For example, the byproduct hydrogen stream from an ethylene cracking plant may have a hydrogen content of 75 mol % whereas the feed to a hydrodealkylation process may require 95 mol % hydrogen. A change in process conditions at a nearby hydroforming plant may create a demand for 99 mol % hydrogen and therefore require the purification of a 90% hydrogen byproduct stream that happens to be available. These examples demonstrate the need for the ability to change selectively from one hydrogen purity to another without having to change equipment specifications.
There are many small to medium size off-gas streams that contain hydrogen and heavier hydrocarbons which are currently being sent to the fuel systems of petroleum refineries. A summary of various hydrogen source streams containing approximate concentrations of hydrogen is published in Oil and Gas Journal, Feb. 6, 1984, p. 111, by Wang et al. In most of the refinery and petrochemical applications where hydrogen is used as a reactant, the desired makeup hydrogen has a purity of about 95%. In order to prevent the build-up of reaction byproducts, such as methane, a portion of the recycle stream is customarily purged. Even though such a stream is relatively small, its concentration of hydrogen represents a loss which must be offset by additional hydrogen make-up.
Several processes have been used and are currently available for upgrading the quality of such off-gas streams. These processes, as described by Wang et al. in the Oil and Gas Journal article of Feb. 11, 1984, include cryogenic separation, catalytic purification, pressure swing adsorption, and membrane separation. Selection of a suitable process depends upon many factors, some of which are the hydrogen product purity that is desired, hydrogen recovery levels, available pressure drop, pretreatment requirements, off-gas composition, impact of reaction products remaining in the hydrogen product, and turndown capability of the selected process.
The bulk of the industrial hydrogen manufactured in the United States uses the process of steam reforming of natural gas according to the equation 2CH4+3H2Oxe2x86x92CO+CO2+7H2. Other processes utilize partial oxidation of resids, coal gasification, and water hydrolysis. Supplying hydrogen by purifying various refinery waste gases is nearly always more economical than hydrogen production by steam reforming. The composition of the raw gas and the amount of impurities that can be tolerated in the product generally determine the selection of the most suitable process for purification.
The impurities usually found in raw hydrogen are CO2, CO, O2, N2, H2O, CH4, H2S, and higher hydrocarbons. These impurities can be removed by shift catalysis, H2S and CO2 removal, PSA process, and nitrogen wash.
An improved extractive flashing version and an improved extractive stripping version of the Mehra Process are respectively described in U.S. Pat. Nos. 4,623,371 and 4,680,042, the entire contents of which are incorporated herein by reference, for separating C2+ hydrocarbons from a nitrogen-rich hydrocarbon gas containing from 3 to 75 mol % nitrogen, the remainder being hydrocarbons.
U.S. Pat. No. 4,832,718, the entire contents of which are incorporated herein by reference, describes a continuous process for separating components of a hydrocarbon gas stream which are selected from the group consisting of hydrogen, nitrogen, methane, ethylene, ethane, higher saturated and unsaturated hydrocarbons, and mixtures thereof by countercurrently contacting the hydrocarbon gas stream with a physical solvent to produce an overhead stream which is rich in at least one of the components and a rich solvent bottoms stream; and by recovering the lean physical solvent from the rich solvent bottoms stream and recycling the recovered stream to the contacting of step. While this process is quite effective, it would be desirable to increase the efficiency of the absorption and the recovery of the desired product from the solvent.
One aspect of the present invention is process and apparatus for separating the components of a hydrocarbon gas feed stream. The hydrocarbon gas feed stream comprises at least a first and second component and typically comprises several components, for example, hydrogen, nitrogen, methane, ethylene, ethane, heavier saturated and unsaturated hydrocarbons and mixtures thereof. Examples of such gas streams include those produced in petroleum refining and in natural gas recovery.
The process comprises contacting the gas stream with a solvent in an absorber to produce a first overhead stream that is enriched in at least the first component and a rich solvent bottoms stream that is enriched in at least the second component. The rich solvent bottoms stream is flash vaporized to recover the second component as a second overhead stream and to produce a lean solvent stream that is returned to the absorber. During the flash vaporization process, the rich solvent bottoms stream is contacted with a stripping gas that is a portion of the first overhead stream. This stripping process enhances the recovery of the second component during the flash vaporization. Because the stripping process removes more of the second component from the rich solvent bottoms, the resulting lean solvent is leaner and more capable of absorbing the second component when the lean solvent is returned to the absorber stage.